LAUSR

The development of on-chip nonlinear optical devices in silicon is of great importance to silicon photonics and silicon chip based quantum information processing technologies. With the aim for a viable solution to overcome the lack of second harmonic generation (SHG) in Si, which is fundamentally limited by its centrosymmetric lattice structure, our work investigates SHG behaviors from Si (100) crystals with silver nanostructures formed following deposition of an ultrathin silver film and subsequent annealing. This study is aided by additional techniques, including x-ray photoelectron spectroscopy for measuring surface band bending, secondary electron microscopy for monitoring surface morphology, and Raman scattering for assessing crystal stress. The resultant Ag nanostructures are found to strongly impact the second order nonlinear polarizations in the Si surface regions rather than the bulk. The SHG intensities are increased following the Ag deposition but reduced below the Si control levels after annealing at 600 and 700 °C, which may be due to charge transfer from Ag to SiO2/Si and/or passivation of interfacial defects. Interestingly, annealing at higher temperatures (800 and 900 °C) leads to the formation of Ag nano-shell structures embedded below the SiO2/Si interface, different from the as-deposited and low-temperature annealing cases with Ag nano-spheroid structures appearing on the surface, and concomitantly, the SHG intensities are recovered and even exceed the level for the as-deposited sample in the p-Si case. The enhanced SHG following high-temperature annealing, particularly at 800 °C, is attributed to a redshift of the localized plasmon resonance of these embedded Ag nano-shells relative to the surface-covered nano-spheroids that leads to localized surface plasmon resonance around the fundamental light wavelength. Other factors including electric-field-induced SHG and the effects of stress or oxide thickness can be ruled out. This work suggests a novel approach to enhance second-order optical nonlinearity in Si through the formation of embedded metal nanostructures.